"The Qt development toolkit is undergoing a major overhaul. The developers behind the project announced the availability of the Qt 5 alpha release this week. It's a key milestone on the path to the official launch of Qt 5, expected to occur later this year." The kind of stuff to read Ars for.

Not on the KDE project, but what leads you to believe that KDE must have the same major version number as the Qt toolkit on which it's based?

While there is no official policy on this, they are the same. Here is why:
1) Because of there policy of maintaining binary compatibility for major numbers and because switching QT major version means broking that compatibility, they will have to increment the revision.
2) They can increment the major revision without switching QT version, but they will not do this unless they will be forced (by new development ideas)

Now lets also look at KDE history:
KDE1 - based on QT1
KDE2 - based on QT2 - major changes, unstable
KDE3 - based on QT3 - *minor changes, stable
KDE4 - based on QT4 - major changes, unstable
So i suspect that:
KDE5 - based on QT5 - *minor changes, stable

* Minor changes does not mean just some small new functionality, but the fact that the major components and architecture remain the same.